Walnut.
These trees get very large, they are a hardwood tree with a long life span. They are squirrel magnets. They drop large amounts of nasty smelling, staining nuts. These trees will ruin the fence, they are planted way to close to it for long term. If they were mine, I'd remove them now before they got any larger. If there are power lines near by sometimes the utility company will remove trees for free, they do here. You may catch a lucky break.
It is also difficult to get other things to grow in the area where they are.
